The present invention relates to hydrostatic power steering systems of the type used to control the flow of fluid from a source of pressurized fluid to a vehicle steering device, such as a cylinder or rotary motor.
A typical hydrostatic power steering system includes a fluid controller of the type having a housing, which defines various fluid ports, and further includes a fluid meter and valving, operable in response to an input, such as the rotation of the vehicle steering wheel. The typical fluid controller also includes an arrangement for imparting follow-up movement to the valving in response to the flow of fluid through the controller valving and the fluid meter to the steering device. The flow of fluid through the controller valving is proportional to the rate at which the steering wheel is rotated.
Steering systems and fluid controllers of the type to which the present invention relates are illustrated and described further in U.S. Pat. No. 3,801,239, assigned to the assignee of the present invention and incorporated herein by reference. The fluid controller of the cited patent was developed in response to a problem which has been referred to as xe2x80x9ctravel limit slipxe2x80x9d. When the steering cylinder attached to the steered wheels of the vehicle reaches the end of its stroke, or hits a mechanical stop, the steered wheels have reached the end of their travel. However, because of possible leakage out of the fluid meter and valving of the fluid controller, continued application of turning force to the steering wheel by the vehicle operator may result in the steering wheel continuing to turn, although typically at a substantially slower rate. The rate of steering wheel rotation in this condition (i.e., against the xe2x80x9cstopsxe2x80x9d) is known as xe2x80x9ctravel limit slip ratexe2x80x9d (TLSR) and is measured in revolutions of the steering wheel per minute.
Those skilled in the art will understand that the above explanation also applies to power steering systems in which the fluid pressure operated device is not a linear steering cylinder, but instead, is a rotary fluid motor, and the invention illustrated and described herein will be considered to apply equally, whether the vehicle steering device is a linear cylinder or a rotary motor.
An attempt to overcome the travel limit slip problem, or at least reduce the TLSR, was illustrated and described in U.S. Pat. No. 5,136,844, also assigned to the assignee of the present invention and incorporated herein by reference. The fluid controller of the cited ""844 patent is of the type referred to as having a xe2x80x9cplugged starxe2x80x9d, i.e., the gerotor (fluid meter) star receives within a counterbore at its rearward face a plug member. Fluid pressure is communicated to the region adjacent the plug, whenever the controller valving approaches its maximum displacement position, thus biasing the opposite end of the gerotor star into frictional engagement with an adjacent housing surface, and preventing further rotation of the meter and the steering wheel. Although the fluid controller illustrated and described in the cited ""844 has been generally successful in reducing the TLSR, the invention of the ""844 patent does involve a substantial redesign of the fluid controller. For example, the valving must be modified, and additional fluid passages provided in both the housing and the endcap, in addition to possible modification of the gerotor star to accommodate the plug.
Another attempt to overcome the travel limit slip problem, and substantially reduce the TLSR, was illustrated and described in U.S. Pat. No. 5,960,694, also assigned to the assignee of the present invention and incorporated herein by reference. In the steering system of the cited ""694 patent, as the steering device reaches the end of its travel, the system control logic generates an appropriate command signal to close a pressure reducing-relieving valve and to open a proportional electromagnetic valve. Fluid pressure increases upstream of the electromagnetic valve, which is then communicated through the valve into the conduit between the fluid controller and the steering device, thus preventing any further rotation of the fluid controller in response to an attempt to turn the steering wheel, thus eliminating travel limit slip.
Although the overall functioning and performance of the steering system of the cited ""694 patent was acceptable, this particular solution to the travel limit slip problem does require extra system logic and extra valves which perform no function, other than to minimize the TLSR.
Certain hydrostatic power steering systems now available are of the type referred to as xe2x80x9cknob controlxe2x80x9d systems, in which there is typically a steered wheel position sensor, and a steering wheel position sensor. Also, such systems generally include valving to port fluid to or from either the high pressure or low pressure side of the system, as a way of compensating for leakage, etc. As a result, the steered wheels are kept proportionately aligned with (in xe2x80x9cregistryxe2x80x9d with) the xe2x80x9cknobxe2x80x9d on the steering wheel, as is generally considered desirable by vehicle operators; thus the term xe2x80x9cknob controlxe2x80x9d.
On a knob control steering system, the valving which ports fluid typically comprises a pair of electrically controlled auxiliary (compensation) valves, with the electrical control signal to the valves being generated in response to the calculation of an error signal. The error signal is calculated to reflect any deviation of the steered wheels from the position commanded by the position of the steering wheel.
Accordingly, it is an object of the present invention to provide an improved hydrostatic (full fluid-linked) power steering system, and an improved fluid controller therefore, which can substantially eliminate travel limit slip.
It is a more specific object of the present invention to provide an improved fluid controller which can accomplish the above-stated object without the need for additional valving and control logic having no other function than to deal with the travel limit slip problem.
It is another object of the present invention to provide such an improved steering system which actively opposes any effort by the vehicle operator to steer xe2x80x9cpast the stopsxe2x80x9d.
The above and other objects of the invention are accomplished by the provision of a full fluid-linked steering system adapted to provide input movement to a pair of steered wheels of a vehicle, in response to manual input to a steering member, said steering system comprising a source of pressurized fluid, a fluid controller, and a fluid pressure operated steering actuator adapted to be operably associated with the pair of steered wheels to provide the input movement thereto in response to manual input to the steering member. The fluid controller defines an inlet port in fluid communication with the source of pressurized fluid, and a control port in fluid communication with the steering actuator. The fluid controller further includes a fluid meter operable to measure fluid flow through the fluid meter, and valve means and means biasing the valve means toward a neutral position. The steering system further comprises compensation valve means in fluid communication with the source of fluid and in fluid communication with the steering actuator, and including valving moveable in response to an external input signal to permit fluid communication between the source and the steering actuator. A load sensing priority flow control valve has an inlet connected to the source of fluid, a control flow outlet port, and an excess flow outlet port.
The improved steering system is characterized by the compensation valve means including a fluid inlet in direct fluid communication with the control flow outlet port of the priority flow control valve. The inlet port of the fluid controller is in direct fluid communication with the excess flow outlet port of the priority flow control valve.
In accordance with a further aspect of the invention, when the fluid pressure operated steering actuator is at the end of its travel, the load sensing priority flow control valve is shifted to a position in which all flow is from the control flow outlet port, through the compensation valve means, and into the conduit between the fluid controller and the steering actuator. This increased pressure downstream of the fluid controller resists the pressure resulting from the torque applied by the operator through the steering wheel and thus to the fluid meter in the fluid controller, such that the operator cannot turn the steering wheel further in the same direction.